Here are the top 12 things that I have found sped up my Mac the best, in no particular order. After you read these, have a look at my results.

12) Update the OS
Later versions of the Mac OS provide bug fixes, features, and speed. If possible, upgrade your machine to Mac OS 9.1, as that is the fastest available Mac OS to date. If your machine cannot handle that, go for 8.6. If you have a fast 040, with a bunch of RAM, OS 8.1 would be your best bet. If you have a slower 68k machine, with not a ton of RAM, go for 7.1 or 7.6. The Mac OS is involved in everything that happens with your Mac, so you might as well get the fastest possible version. (Right now, it's now a good idea to upgreade to Mac OS X if you want more speed)

11) Update your software
Just like the Mac OS, software updates include bug fixes, features, and speed increases (in most cases). Getting the latest version of all your software (my favorite place to check up on this is Versiontracker) will give your Mac an edge when using that software. Hey, you are using this software everyday, you might as well have the best.

10) Trim your extensions
The Mac OS is known for it's customizing abilities. But that comes at the price of RAM and Speed. Every control panel and extension you have installed eats up RAM and CPU cycles. It goes without saying then that if you have fewer extensions, your machine will run faster. With the help of programs like Extension Overload, you can quickly and easily identify the Control Panels and Extensions you don't need. This has to be one of the best ways of speeding up you Mac.

9) Optimize your directories
Optimizing directories with a program such as Disk Warrior gives you a minimal, but noticeable improvement in speed. Not only does it give you a speed boost, but also it keeps problems with your directory from cropping up. This preventive maintenance can save you a ton of time by keeping you from having problems in the future.

8) Upgrade your RAM
If you have the money available, upgrade your RAM as much as you can. While this will not directly transform into a speed increase, when you allocate more RAM to files, you will see the difference extra RAM makes. Not only that, you will be able to open more files, which means more work will get done faster. One thing you should do after installing more RAM is to turn off Memory tests (Mac OS 8.5 and up) by holding down the Command and Option keys while opening the Memory control panel. this will reduce the lengthy start-ups that are attributed to large amounts of RAM.

7) Lower the number of colors
Set the number of colors on your monitor to either 256 or thousands. MacSpeedZone did a test on this, and proved that with fewer colors, the system runs faster. But if you have a G3 or G4, then don't worry about the 256 color setting, go for thousands. You will get a much better picture quality, and the jump from 256 to thousands doesn't effect performance on these machines.

6) Get a Faster Hard Drive
Again, this can only be done if you have the appropriate funds. The Hard Drive is the biggest bottleneck inside of your computer. This is due to it being the only device with actual moving mechanical arms. If you can afford a large SCSI drive, buy that, they are faster than IDE, but IDE is MUCH MUCH cheaper. No matter which kind of drive you buy, make sure to get at least a 7200 RPM drive. 7200 RPM drives read and write data 33% faster than 5400 RPM drives. This will make a huge difference in everything you do. In fact, I feel this speed up my computing experience more than any other upgrade I have ever done!

5) Rebuild the Desktop
You should do this every month, preferably with a utility like TechTool that erases the Desktop file, and rebuilds it from scratch. Over time, the desktop file can get very large, and it consumes a lot of time for your Mac to find what it needs. Everything that goes on with your Mac is related to the Desktop file, so you can see what kind of effect this can have.

4) Defragment your Hard Drive
You should do this whenever your hard drive gets fragmented to .5 percent. I prefer Norton Utilities for doing this, although many other products are available such as Micromat's TechTool Pro 3. When your hard drive is fragmented, it takes longer for your Mac to read, or write to the hard drive. This causes major slowdowns, especially with very large hard drives. Remember 1% fragmentation=10% speed decrease.

3) Virtual Memory
Ever since the introduction of Mac OS 8.0, Apple has been touting a new Virtual Memory Scheme that really speeds things up. Not only that, it makes PowerMac native programs use less RAM. Unfortunately, it seems as if all the bugs haven't been worked out yet. Conflicts still remain, and you will get a speed boost from turning off Virtual Memory. Unless you are low on RAM, or running programs that require Virtual Memory to be enabled, disable this right away.

2) Speed Doubler
Speed Doubler will speed up certain parts of you computer such as file copying, 68k emulation, emptying the trash, and more. Plus, It comes with very useful features such as hotkeys, and the copy agent. However, it doesn't use up RAM, and does conflict with some programs, and doesn't work at all with Mac OS 9.0. If you have an older machine that needs a little boost, Speed Doubler will give you that, just don't expect a miracle.

1) Get a G3 or G4
You really have three options here. Buy a brand new machine, an older used or refurbished machine, or buy and install an upgrade. All three of these methods will significantly speed up you computing experience.